1. Field of Invention
The invention relates to imaging systems and methods. More specifically, the invention relates to imaging systems and methods that may include an alternating pixel arrangement.
2. Description of Related Art
Two-dimensional amorphous silicon (A—Si:H) sensor arrays are well-known devices for real time imaging of incident high energy radiation (see R. A. Street et al., “Large Area Image Sensor Arrays”, in Technology and Applications of Amorphous Silicon, Editor R. A. Street, Springer Series in Materials Science 37, Springer-Verlag, Berlin, 2000, chapter 4, p. 147, for a general description of the structure of the arrays). Such sensor arrays are particularly advantageous for X-ray imaging because they present a relatively large size image sensor array. Each sensor operates on the principal of integrating a charge representative of the quantities of ionizing radiation incident on the sensor. In the direct detection approach, incident high-energy radiation (e.g., X-ray photons) is directly converted to a charge by the sensor (using materials such as Pbl2 or Se). In the indirect detection approach, a phosphor converter absorbs high energy radiation (e.g., X-ray photons) and generates a proportional amount of visible light that is then converted to a charge by the sensor.
To minimize the X-ray dose to patients during medical imaging, there is a need for a—Si:H sensor arrays having the highest possible signal-to-noise ratio. In general, the signal-to-noise ratio of an image sensor array is limited by the electronic noise generated in the array, particularly for imaging conditions when the X-ray dose is low. There are a number of sources of this electronic noise in an image sensor array. A first source is generated by the resistance of the thin-film transistor (TFT) utilized to access the individual image sensors during readout, combined with the sensor capacitance, which gives a noise power of 2 kTC.sub.s (where k is the Boltzmann constant, T is temperature in degrees Kelvin, and C.sub.p is the pixel capacitance). A second source is data line capacitance C.sub.D, which acts on the input of the readout amplifiers of the image sensor array to contribute a noise of N.sub.0+.beta.C.sub.D, where N.sub.0 is typically 200 electrons and .beta. is the noise slope of about 15 e.sup.−/pF. A third source is generated by thermal noise of the data line resistance, which can be represented by 4 kTR.sub.D.DELTA.f, where R.sub.D is data line resistance, and .DELTA.f is typically 1 MHz, but depends on the speed of the readout amplifier. A fourth source of electronic noise is line-correlated noise that is capacitively coupled from the gate and bias line power supplies to the data line, and is proportional to the data line capacitance.
Of the various sources of electronic noise in large area and high-resolution image. sensor arrays, data line capacitance tends to be the largest noise source, since it is proportional to the very large number of pixels (i.e., individual sensors and associated TFTs) coupled to each data line. For a typical array, the data line capacitance per pixel is 30-50 fF, which gives a total capacitance of about 100 pF, and an amplifier noise of about 1700 electrons. The kTC noise of each sensor is typically in the range of 300-600 electrons, depending on the size of the pixel, and the thermal noise of the data line can be made small by choosing a low resistance metal and limiting the amplifier bandwidth. The line-correlated noise can be minimized by very careful design of the power supplies, but for very large arrays is about 1000 electrons.
With these parameters, data line capacitance becomes the most significant source of electronic noise, and a reduction in the data line capacitance could significantly reduce the electronic noise, which would also reduce the requirements for very high performance readout amplifiers and very low noise power supplies. Most importantly, reducing the noise produced by reducing data line capacitance would increase the signal-to-noise ratio of the sensor array, thereby facilitating medical imaging using lower X-ray doses.
While the sensor capacitance can be a source of noise, it also serves an essential role in keeping the bias voltage across the sensor constant. As photocurrent is integrated across the sensor, the bias voltage drops until the sensitivity goes to zero. In addition, the voltage difference on adjacent pixels increases, which might cause leakage between adjacent pixels. Since imaging applications demand high dynamic range, the sensor capacitance is a crucial design criteria. For indirect-detection arrays, a thin photoconductor might provide sufficient pixel capacitance C.sub.p to accumulate the charge. The sufficiency depends on the material properties of the photoconductor, primarily the absorption length of visible light. Amorphous silicon has a short absorption length, and typically a thin layer of this material can act as the pixel capacitor as well as the sensor. Crystalline silicon, in contrast, would need to be thick and would not have sufficient intrinsic capacitance. For direct-detection, however, the absorption of length of x-rays in the clinically useful energy range is large even for high-atomic number materials.
The photoconductor layer must be thick in this case in order to ensure high sensitivity. Consequently, the intrinsic capacitance of the photoconductor is unlikely to be sufficient. Adding a capacitor to each pixel in parallel with the sensor capacitance will boost the capacitance C.sub.p. This addition requires an additional ground connection to each pixel. If the ground connection is disposed parallel to the gate connections, then the data line must cross over a wire on the gate metal level double the amount of times. This configuration may add to the capacitance coupling between the data line and ground. If the ground connection is disposed parallel to the data connections, glitches from the switching signal may couple onto the sensor, introducing additional line-correlated noise.
Accordingly, what is needed is an image sensor that significantly reduces data line capacitance, and possibly reduce data line resistance, to significantly increase the signal-to-noise ratio of the sensor array.